Lignocellulosic biomass derived from plant cell walls is the most abundant raw material for biofuels and renewable chemicals production. Hemicellulose comprises about 30% of the total weight of lignocellulosic biomass. The predominant form of hemicellulose are heteroxylans, which contain a backbone of 1,4-linked β-D-xylose polymers and various decorations of arabinose, ferulic acid, galactose, glucuronic acid, and acetyl ester. In contrast to cellulose, hemicellulose components are readily depolymerized into short oligomers and released into the liquid phase during pretreatment. It is of great interest to convert the released hemicellulose components into fuels or other value-add chemicals for building an economical biomass conversion process.
It has been estimated that there are ten times more microorganisms than human cells in a healthy adult (Savage et al., 1977). The symbiosis between the microbiome and human organs is increasingly recognized as a major player in health and well-being. Xylooligosaccharides and xylitol, both derived from hemicellulose, can benefit gut flora and oral flora, respectively.
Xylooligosaccharides (XOS, also called xylodextrins) are naturally occurring oligosaccharides, found in bamboo shoots, fruits, vegetables, milk and honey (Vazquez et al., 2000). Industrial scale production of XOS can be carried out with much less expensive lignocellulosic materials by hydrothermal treatment or enzymatic hydrolysis (Aachary et al., 2011). A broad range of applications of XOS have been demonstrated, including as functional food, prevention and treatment of gastrointestinal infections, animal feed for fish and poultry, agricultural yield enhancer and ripening agent, and as active agents against osteoporosis, pruritus cutaneous, otitis, and skin and hair disorders. In the current market, the most important applications of XOS correspond to ingredients for functional foods as a prebiotic, or formulated as synbiotics (Schrezenmeir et al., 2001). A prebiotic is a “nondigestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon and thus improves the host's health” (Gibson et al., 1995). XOS has been shown to promote beneficial bacteria Bifidobacterium adolescentis growth in vitro (Okazaki et al., 1990) and in vivo (Hsu et al., 2004). It has been estimated that the prebiotics market will reach $4.8 billion by 2018, and the current largest producer (Shandong Longlive Bio-technology Co., Ltd.) has a 10,000 metric ton annual capacity of XOS.
Xylitol is another hemicellulose-derived compound beneficial to human health. For many bacteria and yeasts, the uptake of non-utilizable xylitol interferes with hexose utilization, which helps the human body to rebuild a healthy microbiome. Xylitol has been used to prevent middle ear infections (Azarpazhooh et al., 2011) and tooth decay (Maguire et al., 2003; Makinen et al., 1992). In addition, xylitol possess 33% fewer calories but similar sweetness compared to sucrose (Hyvonen et al., 1982) and has been widely used as a substitute sweetener. While chemical hydrogenation of xylose remains the major industrial method of xylitol production, microbial fermentation has become more popular in the newly built plants due to lower conversion cost. The annual production of xylitol is between 20,000 and 40,000 tons, based on a 2007 report (Granstrom et al., 2007). There exists a need for improved methods of producing xylooligosaccharides and related compounds, such as xylooligosaccharides with xylitol components.